There are many instances when it is desirable to run a tool string downhole into a borehole with a lower end portion of the tubing string being opened to the flow of well fluids from the borehole so that no differential in hydrostatic head is developed. In a producing well, it may be desirable to reperforate the existing producing formation or to perforate another production zone within the well. In such a situation, a hydrostatic head of drilling mud is used to maintain a bottomhole pressure that is greater than the formation pressure to insure that the well is under control at all times and thereby prevent any blowout. If a sufficient hydrostatic head were not established, the well could start "kicking" during the new perforating.
The general tubing conveyed perforation technique includes a tubing string with a closed vent assembly and perforating gun. The tubing string is run into the well substantially dry with only a small amount of fluid in the bottom of the string to cushion the impact of a bar dropped through the string to detonate the perforating gun. Thus, the vent assembly in the tool string is run into the well in the closed position. However, where it is necessary to maintain the hydrostatic head as in a producing well, the lowering of a dry tubing string into the well would reduce the hydrostatic head so as to possibly cause the loss of control over the well. Thus, it is desirable to run the tubing string into the well "wet" with a vent assembly open whereby well fluids can run into the tubing string to maintain the hydrostatic head. Further, if the well should start "kicking", the open vent assembly permits circulation down through the tubing and into the well to provide further means to kill the well at any time.
For example, often in a dual formation well where the production fluid from the two formations can be co-mingled, the lower zone is perforated and tested and then gently killed with calcium chloride and water such that the completion will not be damaged. If one were to go back into the well with dry tubing, there would be no means to maintain the hydrostatic head or to circulate through the tubing string such that the lower formation would start producing before the dry tubing string reached the location of the upper formations to be perforated. This would occur due to the reduction of the hydrostatic head to a value lower than the formation pressure causing the lower formation to start producing.
The present invention provides a means whereby a perforating gun can be run downhole on the end of a tubing string along with a packer actuated vent assembly held in the open position and which can be subsequently moved to the closed position upon the setting of the packer. Additionally, there is another vent assembly included in the tool string below the packer which can be moved from the closed position to the open position at any subsequent time such as just prior to the detonation of the perforating gun. This unique combination enables an extremely large casing type perforating gun to be run downhole with the tubing string open to the flow of well fluids whereby there is a zero back pressure on the tubing string. After the tool string has been positioned downhole in the borehole, the interior of the tool string can be isolated from the fluids contained within the casing annulus by closing the packer actuated vent assembly. Once the gun is suspended downhole adjacent to the production formation, the second vent assembly is moved into the open position and the gun is detonated at some subsequent time.
Further, once the packer is in position and can be set, the present invention provides the option of lowering the hydrostatic head in the tubing string by displacing the well fluids in the tubing string with another fluid such as nitrogen. As the nitrogen is pumped down the tubing string, the well fluid in the tubing string are displaced through the open packer actuated vent assembly of the present invention. Once the desired hydrostatic head is reached, as for example to obtain an underbalance, the packer actuated vent may be closed and the nitrogen bled off to obtain the desired hydrostatic head in the tubing string to provide the desired pressure differential for backsurging. The underbalance or pressure differential can also be achieved by swabbing the tubing string dry after the packer actuated vent assembly has been closed.
In the prior art, sliding sleeves actuated by wireline have been used to permit flow into the tubing string. Such a sliding sleeve is manufactured by Baker Oil Tools. However, such sliding sleeves are not dependable and do not always seal. Further, the wireline can be blown out of the hole and become tangled. Also, it is cheaper to use a vent assembly in the tool string which can be actuated by the setting of the packer than use a wireline operated sleeve.